Crystal structure of Cr-bearing Mg3BeAl8O16, a new polytype of magnesiotaaffeite-2N'2S.

The crystal structure of a new polytype of magnesiotaaffeite-2N'2S, ideally Mg3BeAl8O16 (trimagnesium beryllium octa-aluminium hexa-deca-oxide), is described in space-group symmetry P-3m1. It has been identified in a fragment of a mineral sample from Burma (Myanmar). The new polytype is composed of two Mg2Al4O8 (S)- and two BeMgAl4O8 (N')-modules in a stacking sequence N'SSN'' which differs from the N'SN'S-stacking sequence of the known magnesiotaaffeite-2N'2S polytype. The crystal structure can be derived from a close-packed arrangement of O atoms and is discussed with regard to its polytypism and its Cr(3+) chromophore content.

Micro-Raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins.

Three bronze samples created by CNR-ISMN (National Research Council-Institute of Nanostructured Materials) to be similar to Punic and Roman coins found in Tharros (OR, Sardinia, Italy) were studied to identify the corrosion products on their surfaces and to evaluate the reliability of the reproduction process. Micro-Raman spectroscopy was chosen to investigate the corroded surfaces because it is a non-destructive technique, it has high spatial resolution, and it gives the opportunity to discriminate between polymorphs and to correlate colour and chemical composition. A significant amount of green copper hydroxychlorides (Cu(2)(OH)(3)Cl) was detected on all the coins. Their discrimination by Raman spectroscopy was challenging because the literature on the topic is currently confusing. Thus, it was necessary to determine the characteristic peaks of atacamite, clinoatacamite, and the recently discovered anatacamite by acquiring Raman spectra of comparable natural mineral samples. Clinoatacamite, with different degrees of order in its structure, was the major component identified on the three coins. The most widespread corrosion product, besides hydroxychlorides, was the red copper oxide cuprite (Cu(2)O). Other corrosion products of the elements of the alloy (laurionite, plumbonacrite, zinc carbonate) and those resulting from burial in the soil (anatase, calcite, hematite) were also found. This study shows that identification of corrosion products, including discrimination of copper hydroxychlorides, could be accomplished by micro-Raman on valuable objects, for example archaeological findings or works of art, avoiding any damage because of extraction of samples or the use of a destructive analytical technique.

Structures of the pseudo-trigonal polymorphs of Cu2(OH)(3)Cl.

The crystal structure of Cu2(OH)(3)Cl has been determined using two natural samples with almost ideal stoichiometry. While one of the samples exhibits a twinned clinoatacamite structure, the other sample is characterized by the appearance of additional weak diffraction maxima at half integer positions of h and k. Structure refinement was carried out with the space group P1. The relationship between the triclinic phase, clinoatacamite, paratacamite and the herbertsmithite structure is discussed in terms of symmetry as a function of Cu concentration.

Cu3MgCl2(OH)6 and the bond-valence parameters of the OH-Cl bond.

The title compound is a new mineral consisting of sheets made from edge-sharing Cu(OH)4Cl2 and Mg(OH)6 octahedra. The sheets are only weakly linked by O-H-Cl bridges, giving rise to perfect cleavage along (001) and to strong texture effects in powder diffraction measurements. Owing to low bond-valence sums at the Cl sites of Cu3MgCl2(OH)6 and of several related compounds, bond-valence parameters for the H/Cl pair have been refined to R0=1.336 and b=0.53 A based on the crystal structures of selected hydroxychlorides.